FIG. 1 shows the structure of the prior art atomic force microscope (AFM). In this figure, a sample 1 is placed on a z-scanner 2 which displaces the sample vertically or in the z-direction. The z-scanner 2 is placed on an xy-scanner 3 which moves the sample within the xy-plane perpendicular to the z-direction. The z-scanner 2 and xy-scanner 3 are driven in the z-, x- and y-directions by piezoelectric devices, for example. A scanning signal produced by an xy-scanning signal generator 4 is supplied to the xy-scanner 3.
A cantilever 5 consisting of a resilient body having one fixed end is located above the sample 1. A piezoelectric device 7 for applying vibration is mounted close to the fixed end of the cantilever 5. A probe 6 whose tip faces the sample 1 is mounted to the front end of the cantilever 5. The top surface of the cantilever 5 is a reflecting surface to which laser light L emitted from a laser 8 is directed. Light L' reflected from the cantilever 5 reaches a photodetector 9 where the light is detected. The photodetector 9 consists, for example, of bi-cell photodiodes and detects variations in the position of the reflected light L' caused by vibrations of the cantilever 5.
The output signal from the photodetector 9 is fed to an analog amplifier 10 incorporating an automatic gain control (AGC) circuit 11. The output signal from the amplifier 10 is supplied to a band-pass filter 12 which passes only frequencies close to the resonance frequency of the cantilever 5. The output signal from the filter 12 is furnished to a phase-adjusting circuit 13 and then to the piezoelectric device 7.
The AC signal from the photodetector 9 which is modulated with the resonance frequency of the cantilever 5 is converted into a voltage signal by a frequency-to-voltage (F/V) converter circuit 14. A reference voltage comparator 15 produces the difference between the voltage signal V.sub.fv from the frequency-to-voltage converter circuit 14 and a reference voltage signal V.sub.fvo. The difference signal is sent to the z-scanner drive circuit 17 via the low-pass filter 16. The image-creating circuit 18 creates an image according to the difference signal extracted via the filter 16.
In the above-described structure, the cantilever 5 is deflected periodically by the piezoelectric device 7 and thus vibrated. In this way, the front end of the cantilever 5 is moved up and down. This results in variations in the position of the reflected light L' incident on the photodetector 9. The output signal from the photodetector 9 varies according to the varying position. FIG. 2(a) shows the waveform of the output signal from the photodetector 9. In the diagram of FIG. 2(a), time (t) is plotted on the horizontal axis while the z-position of the probe 6 is plotted on the vertical axis.
Those frequencies of the output signal from the photodetector 9 which are close to the resonance frequency of the cantilever 5 are extracted by the band-pass filter 12. The phase is adjusted by the phase-adjusting circuit 13 to produce positive feedback. The output signal from the phase-adjusting circuit 13 is supplied as a driving signal to the piezoelectric device 7. In this way, a self-oscillating loop for positive feedback is formed. As a result, the cantilever 5 keeps vibrating at its resonance frequency.
The AGC circuit 11 controls the gain of the amplifier 10 so that the amplitude of the output signal from the photodetector 9 is maintained constant. Since the controlled output signal from the amplifier 10 is supplied to the piezoelectric device 7, the amplitude of the vibration of the cantilever 5 is held constant. For example, if the amplitude of the vibration of the cantilever 5 increases or decreases for some cause to thereby vary the amplitude of the output signal from the photodetector 9, then the AGC circuit detects the increase or decrease in the amplitude of the output signal from the photodetector 9 and increases or reduces the gain of the amplifier 10 in such a way that the variation in the amplitude is canceled out. This varies the driving voltage of the piezoelectric device 7, thus canceling out the variation in the amplitude of the vibration of the cantilever 5.
When the cantilever 5 keeps vibrating at a constant amplitude in this way, if the sample 1 is brought close to the probe 6 until an atomic force is exerted between them, and if the xy-scanner 4 makes a two-dimensional scan in the x- and y-directions, then the resonance frequency of the cantilever 5 is apparently decreased under the influence of the gradient of the atomic force acting on the cantilever 5 according to the distance to the sample 1. The cantilever 5 vibrates at the decreased frequency. This frequency drops as the distance between the sample 1 and the probe 6 decreases. Conversely, if the distance increases, the frequency is increased. If the distance is so great that the atomic force is negligibly small, then the frequency becomes coincident with the resonance frequency of the cantilever 5.
The output signal from the photodetector 9 representing the vibration is converted into the voltage V.sub.fv corresponding to the frequency of the vibration by the frequency-to-voltage converter circuit 14. The comparator 15 produces the difference between this voltage V.sub.fv and the reference voltage V.sub.fvo to the z-scanner drive circuit 17 via the filter 16. It follows that a feedback control loop for controlling the distance between the probe 6 and the sample 1 according to the frequency is formed. The distance between the probe 6 and the sample 1 is maintained at a given value determined by the reference voltage V.sub.fvo,
For instance, if the surface of the sample 1 is uneven, and if the distance between the probe and the sample decreases in the direction to lower the vibration frequency of the cantilever 5 as a result of a two-dimensional scan made by the xy-scanner, then the output voltage V.sub.fv from the converter circuit 14 drops, thus increasing the difference signal. The z-scanner 2 immediately lowers the sample 1. This, in turn, provides such feedback that the distance to the probe 6 is increased. Consequently, the distance between the probe 6 and the sample 1 is kept at the given value determined by the reference voltage V.sub.fvo. Since this control operation is constantly performed, the feedback signal (difference signal) supplied to the z-scanner drive circuit 17 corresponds to the unevenness or topography of the sample surface. This feedback signal is accepted as an image signal into the image-creating circuit 18 in association with the two-dimensional scan made by the xy-scanner. An image is displayed according to the accepted image signal. In this way, the topography of the sample surface can be imaged according to the atomic force.
If the amplitude of the vibration of the cantilever 5 changes for some cause or other, then the amplitude of the output from the photodetector 9 varies. The AGC circuit 11 varies the gain of the amplifier 10 in such a way that the amplitude of the output from the amplifier 10 is maintained constant. In order to perform this control operation stably, a sufficiently large time constant is necessary. Furthermore, as the gain varies, the phase of the output from the amplifier 10 changes. In this manner, some factors impede stable self-oscillation showing good response. If the oscillation is stable (i.e., the amplitude is regulated), the response follows variation of the frequency caused by the unevenness of the sample surface with delay. Hence, an optimum feedback signal tends not to be produced. This directly deteriorates the quality of the obtained microscope image and the resolution.
The output signal from the photodetector 9 is directly sent to the frequency-to-voltage converter circuit 14. Since this signal is feeble, good frequency-to-voltage conversion cannot be done. As a result, the optimum feedback signal is not obtained. In consequence, the z-scanner 2 is not appropriately driven and the sample may not be imaged well.